METHOD AND APPARATUS FOR MEASURING GEOPHYSICAL DATA

Abstract: Simultaneous measurements from a plurality of positions are made of geophysical data particularly electrochemical potential and resistance of the ground enabling contaminants such as oil or chemicals to be located beneath the surface or leakage in under surface sealing walls to be detected. The apparatus comprises microprocessor means (12) connected by cable means (13) to a plurality of measuring sensors (15), each sensor being capable of measuring a ground potential at a position in the array at which it is located relative to a reference potential located at a base reference sensor (16, 17), the processor being programmed to control and record substantially simultaneous measurements from all the sensors, the processor being programmed to identify the information from each individual sensor and relate this to the position of the sensor in the array, and means arranged to use the data from said measurements to produce contour and/or graphic profile displays of the variation of the ground resistance and/or electrochemical potential over the target area. The apparatus includes means for introducing into the ground at the base reference sensor a known current and calculating from recordings made substantially simultaneously with the introduction of the current, the ground resistance.

Description: METHOD AND APPARATUS FOR MEASURING GEOPHYSICAL DATA FIELD AND BACKGROUND OF THE INVENTION This invention relates to a method, and apparatus for measuring and recording geophysical or geoche ical parameters of the geographical surface or near surface. Within the near surface layers of the earth the interaction of the interfaces of the various grains of sand, soil or the like or interfaces of various rock types combined with any residual water or other liquids or materials creates a resistance and/or potential across these various material surfaces. An object of this invention is to derive useful information from measuring such geophysical data at an array of points on or in the ground. Electrochemical or self potential that exists between different points in the near surface is a well known geophysical parameter and apparatus for measuring it is known; equally apparatus for measuring the reistance or resistivity of the ground is well known. However, the same apparatus for measuring natural or self potential is not normally also used for measuring resistivity and the data provided by making such combined measurements is not conventionally regarded as data which can be particularly usefully combined and applied. In particular such data is usually measured at different locations at successive times and is subject to natural variations of the data with time, temperature, weather, and man-made environmental noise. Apparatus for measuring the resistivity of the ground is known for example from GB-A-2180070, EP0426563 and EP-A- 0110750. However these apparatuses are basically concerned with measuring resistivity at considerable depths in bore holes. The currents and voltages injected are alternating and of the order of tens of volts or they would not penetrate to sufficient depths and the voltage measuring sensors are not sensitive enough to measure the natural ground potential in the surface and near surface (down to tens of metres). The natural potentials are normally of the order of nanovolts to hundreds of millivolts. GB-A-2253912 and WO-A-90/05923 disclose a horizontally arranged set of electrodes in contact with the surface or near surface but they pass a consistent alternating current between a first two electrodes while the potential is measured between a second two electrodes. Sequential measurements are taken between different pairs of second electrodes using the same current between the first electrode and a map of figures derived from these measured potentials (basically representing resistivity) may be drawn. SUMMARY OF THE INVENTION The method invented is a combined geophysical measurement in a horizontal linear or areal mode on the surface or in a vertical mode in the near subsurface to evaluate the geoelectrical effect (resistivity) of different geological and geochemical bodies together with their electrochemical bodies together with their electrochemical response to the passive DC-field of the upper earth crust. The computerised registration of different electrical properties with the same field equipment without moving sensors, in regular or irregular, horizontal or vertical arrays allows after intensive data processing and correlation a combined data set (anomaly display) of all surface and subsurface geological and geochemical events. The active measurement (surface resistence) allows differentiation of geological surface bodies even under very complex situations and produces all values necessary for a true correction of the measured data set with the electrochemical response of geological and artificial bodies summed up with the surface influences. Only this combined measurement with the same sensors (geographical position) allows, through correlation and correction, the separation of surface and subsurface information. Accordingly, in one aspect, the present invention provides a method of collecting and recording geophysical data which comprises recording substantially simultaneously a plurality of first type measurements of the ground potential at an array of selected positions in a target area, each measurement being relative to a common base reference, recording substantially simultaneously a plurality of second type measurements of ground potential using the same positions in the same array and with the same base reference, the first type measurements being made substantially coincidentally with introducing a known current into the ground, and correlating the recorded measurements to produce one or more displays of the variation of electrochemical potential and/or resistance of the ground over the target area. The first type measurements are preferably used to calculate resistance or resistivity of the ground. It is immaterial whether the first type measurements are made before or after the second type measurements. In the preferred form of the invention the non polarised measuring sensors at the positions are arranged in a two dimensional array or grid over a surface area of the ground and the display will include a pair of contour maps, one of resistance and one of electrochemical potential over the substantially horizontal target area. Interpretation of the two data sets results in a new geophysical 2-D display, giving an altered aspect of the near surface. We have found that from such displays useful information on the make-up of the sub-surface can be deduced, enabling contaminants such as oil or chemicals to be located beneath the surface without the need for exploratory drilling. The sensors or one or more base point reference sensors may be located linearly at selected spaced positions in a bore hole or surface hollow and the displays will then include graphic profiles of the resistance of the ground and/or electrochemical potential as it varies in substantially vertical planes through the near surface ground less than 10 metres) in the target area. The resistance is calculated from a simple V=IR equation, since the voltage V is measured by the sensors and the current value I is known. The calculations can include adjustments for the electrochemical potential and the different distances of the sensors from the base reference. Preferably several measurements of the first type and of the second type are taken using the same sensors but using a different series of base reference sensors or reference sensor lines in succession. These base reference sensors may be outside the target area or may comprise some or each of the sensors in the target area allocated, in turn, to serve as a base reference measurer. With advantage the sensors are divided into groups, each group associated with an individual data recording and transmission unit. Initially the potential of each sensor relative to others of the sensors in its group may be measured and the results are surveyed to ensure that each sensor is properly connected both to the control system and to the adjacent ground surface. The finer the spacing of the sensors in the grid or array, the higher the quality of the data and the more precise the contour or profile mapped displays will be. The array spacings are therefore determined by the required quality of the final data display. For example the sensors may be spaced from 0.55 meters to 20 meters apart over the target area or over the perimeter of the target area. There may be from 20 to 4000 sensors in an array. Taking the measurements at different times at different parts of the target area would produce inaccurate results, because the induced fields vary substantially with time, dependent for example on lunar effects, weather effects, background electrical noise effects and other natural and environmental changing conditions. By taking the recordings substantially simultaneously at all sensors some of the errors produced by these effects are eliminated. By taking a series of recordings using different base references and reviewing the results, other transient anomalies can be observed by a skilled user of the data, and the data including these anomalies rejected. Corrections can be introduced for the natural variation of the data in different magnetic directions. Another use of the invention is to locate any breach of or leakage through vertical or horizontal subsurface sealing walls intended to prevent or reduce the flow of water therethrough. Such walls may be single walls, multiple parallel walls or a matrix of walls defining a series of chambers. Walls are commonly built around areas where waste is stored to prevent contaminated water leakage. As the walls are built by injecting concrete into the ground they are of uneven thickness and holes may result. Locating the position of such a hole without digging can be extremely useful. A series of sensors are located in a pattern at various points along the wall or walls either on one side of a wall or in a pattern straddling the wall or walls. Data is recorded from the sensors as before, with the base reference sensor either on the surface or lowered into a convenient bore hole or well. Profile maps of the resistivity will tend to show a reduced resistivity in the area of a wall hole. The leak flow pattern may be artificially varied or enhanced by pumping water to or from one side of a wall or an area between or outside the walls and taking recordings before and after the variation. A tracer material such as animal salt may be introduced into the ground water. In another aspect the invention provides apparatus for measuring and recording ground resistance and electrochemical potential between a reference position and a plurality of measuring positions in an array in a target area, comprising microprocessor means, connected by cable means to a plurality of measuring sensors, each sensor being capable of measuring a natural or self potential at a position in the array at which it is located relative to a reference potential, and measuring potential substantially coinciding with injection of a current pulse the processor means being programmed to record substantially simultaneously the measurements from all the sensors, the processor means being programmed to identify the information from each individual sensor and relate this to the. position of the sensor in the array, and means arranged to use the data from said measurements to produce contour and/or graphic profile displays of the variation of the ground resistance and/or electrochemical potential over the target area. Preferably the apparatus is arranged to introduce into the ground at the base reference point a known direct current pulse and to take simultaneously from all the sensors a recording of the ground potential substantially coincidentally with the introduction of the current. The apparatus is programmed to calculate data concerning the ground resistance from such voltage measurement taken by the sensors relative to the base reference. The calculations of resistance can include compensation for the measured electrochemical potential of the ground. Preferably the microprocessor is programmed to take a succession of such measurements from all the sensors relative to a plurality of base reference sensors or sensor groups at different positions. With advantage the apparatus includes a plurality of data transmission and recording units, each connected by cable to a central data receiving and processing means, and to each of which units a group of the sensors is connected. These units make the simultaneous recordings from their associated group of sensors and transmit their semi processed data, in turn, to the central data processing means, where it is correlated to produce the required data from which the displays/maps are made. The invention extends to maps or displays produced by the method or apparatus. At least one of such maps may be printed on transparent material so that it can be overlaid on another such map. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of method and apparatus for measuring geophysical data will now be described, by way of example only, with reference to the accompanying drawings of which:- Figure 1 is a schematic block diagram illustrating apparatus to be used for measuring, Figure 2 is a diagrammatic graph illustrating changes in potential with time, Figure 3 is a contour map showing resistance plotted over the a target area, Figure 4 is a contour map showing electrochemical potential plotted over the same target area, Figure 5 is a diagrammatic illustration of another arrangement of the apparatus, Figure 6 shows a diagrammatic section through two parallel walls with adjacent sensors, Figure 7 shows a plan view of a single wall and a plurality of sensors, Figure 8 shows a plan view of a matrix of walls and a plurality of sensors, Figures 9 and 10 are contour maps of ground potential adjacent a wall, and Figure 11 is a diagrammatic profile map of resistivity measured by different sensors spaced along the ground adjacent a wall. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Referring to Figure 1, the apparatus comprises a central control, receiving and processing microprocessor 12 connected by cable 13 to a series of data recording and transmission units 14 arranged in series. Each unit 14 has connected thereto four sensors 15, of known form, capable of measuring the ground potential at a location in which it is placed in contact with, or inserted into, the ground. The sensors are arranged to measure voltages in the range nanovolts (10 volts) to 1 volt, although the voltages measured will normally be tens of millivolts. One or more base point or reference sensors, two of which are shown at 16 and 17, are connected to the units 14. In use each unit receives a continuous signal from one selected reference sensor and each of the sensors 15 connected thereto. Although the diagram illustrates the base point sensors as individually connected to the units 14, they will, in practice, be connected through the cable 13. Each unit is programmed, on receipt of a command signal from the central processor 12, to make substantially simultaneously a recording of the potential values measured by each of its associated sensors 15 relative to the base point. The command signal is issued to each unit simultaneously The control processor will be programmed to instruct recordings to be taken, in turn, relative to different ones of the remote reference sensors 16, 17. Alternatively or in addition the control processor may be arranged to allocate one or more of the sensors 15 to serve as a reference sensor and may instruct recordings to be taken relative to some or each of the sensors 15, in turn, acting as a reference sensor(s) . In use, the cable 13 is spread over the surface of a target area to be analysed in a two dimensional configuration, for example an undulating or zig-zag configuration. The sensors 15 attached to each unit 13 are spaced out, preferably so that the sensors are arranged in a well ordered grid or array with equal spacings from one another. For example each sensor may be located at a 10 metre spacing from each adjacent sensor. The cables 18 connecting two of the sensors associated with one unit 14 may be longer than the cables 19 connecting the other two sensors in that group, for example the cables may be respectively 6 metres and 16 metres, with the units 14 spaced apart 30 to 35 metres. Whatever spacing is chosen, the sensors will be marked so that they are always arranged in a predetermined order in the array and the central processor will be supplied with information concerning the spacing and location so that the processor can identify the information from a particular sensor with a particular location on the ground. By taking readings relative to a plurality of spaced base reference points over the array of sensors, a set of data can be built up relative to the whole area of the array. The apparatus includes means for introducing a direct current pulse of known microamp value, for example 50 microamps, into the ground via one or more base reference sensors. This will be introduced from a direct voltage source normally of 0-lOV, for example about 600mV. The control unit is programmed to instruct simultaneous measurements to be recorded, as before, immediately after or coincident with the introduction of this current. This will record a voltage higher than that of the electrochemical potential. This allows a calculation of the resistance to be made using the simple V=IR relationship with adjustments. As shown diagrammatically in Figure 2, the self potential V as measured relative to a reference potential at any sensor, varies with the time T. If a current pulse, as shown at I, is introduced into the ground and a measure of potential is taken substantially coincidentally therewith, the change in potential as a result of the current can be measured. For example if measures of potential are taken at times Tl and T2 the difference between these measures will be the result of the introduction of the known current value and the resistance value of the ground between the sensor and base point can be calculated. The data from the individual sensors passes through a filter to eliminate as many of the unwanted signals as possible. The data is temporarily stored and any offset data determined and removed in the units 14 and is transformed from analogue form into a digital value within the units 14. The digital data is analysed and a value of the resistance of the ground adjacent each individual sensor with respect to a distant sensor or pre-allocated sensor is determined. This data value is combined with a status and command srring within the data transmission units 14 before being sent back to the controlling central processor for display and storage. The data transmission units 14 are controlled by the central control unit 12. The data received by the central processor is stored in a magnetic media and can be simultaneously displayed either in graphical form or numerical form for the operator's quality control and amendment. The data received by the central computer is later processed using standard off-the- shelf type software. Processing procedures can include quality control to ascertain if unwanted data has been included into the data set. Once this has been completed the data is further processed to produce contour, or other maps of the measured area. Figures 3 and 4 show sample contour maps showing respectively the resistance and electrochemical potential of the near surface over an area covered by the sensors and with the contours extrapolated slightly beyond the array of sensors. The dots on the contour map mark the position of the sensors. From these maps, a skilled interpreter can locate areas which show various anomalies and which can be drilled for following cleaning programmes if necessary. A further embodiment of the invention is seen in Figure 5. A bore hole 20 is formed in the ground in the target area and a first set of sensors 21 connected by cable 23 are located in the bore hole in a linear array and pressed into engagement with the ground forming the wall of the bore. With a minimum of one a second set of sensors 22 are arranged in a two dimensional or linear array engaged with the ground on its surface. A series of simultaneous measurements of restance and/or electrochemical potential are then made as before using one set of the sensors as the array and one of the sensors of the other set as a base reference. A series of such readings may be taken with different ones of the sensors as the base reference sensor. The data sets from these figures may be used to produce contour maps or graphic profile maps in a number of different vertical planes through the near surface of the ground or 3D data blocks of information. This allows the user to target changes on the variations in the near surface from the down hole sensors up to the surface sensors. Figure 6 shows a pair of vertical, parallel, under surface sealing walls 30, 31 with the ground water level 32 different on each side of each wall. A plurality of sensors 33, connected to each other and a processor (not shown) as before, is located in an array on the surface adjacent the walls. The array can be on one side or both sides of the or each wall (there may be a single wall) in any suitable pattern. One or more base reference sensors 34, 35 on the surface or in a bore 36 is also provided as before, and simultaneous measurements are recorded and processed as before described. The data can be usefully used to locate the position(s) of leakage through the wall or walls. Figure 7 shows a single itfall 37 with sensors 38 arranged in one form of staggered array and a base sensor 39, all connected to a processor (not shown). Figure 8 shows a matrix of walls 40 defining chambers 41 and an array of sensors 42 connected to a processor (not shown), with a base sensor or means for using some of the sensors 42, in turn, as a base sensor. (a) Measurements are made in what can be regarded as the passive mode when water is flowing through the wall, if there is a leakage, under natural means and the ground water table is stable i.e. normal flow to the lowest level in the surrounding area such as a stream or river. (b) Measurements are made when an artificial ground water table is created by pumping away some of the water on either side of the wall creating an artificial flow through or under the wall thereby increasing the flow potential in the subsurface which is recorded. (c) When the situation is parallel walls, measurements are made when an artificial ground water table is created by pumping away some of the water on either side of the wall or from between the walls creating an artificial flow through or under the wall thereby increasing the flow potential in the subsurface which is recorded. (d) If a matrix of walls are encountered then the water must be pumped out of or into the chambers that are suspect creating the unnatural flow. (e) All of the above methods rely upon the natural ground water to generate the flow potential but, an alternative method is to use a tracer material, e.g. salt where permissable, to highlight the changes in the subsurface by either altering the conductivity of the ground water or the addition of a subsurface material differing in makeup to the surrounding medium. This tracer material can either be allowed to distribute itself into the subsurface or ground water or can be influenced by pumping as in sections (a) to (d). Figure 9 shows one example of a contour map (normally produced in colour) of ground potential measured by an array of sensors 45 spaced two metres apart from one another and located in an array on the ground surface on opposite sides of an undersurface wall indicated diagrammatically at 46. In this example the wall extends through the middle of the map which extends for a fifty metre length of wall. Measurements of ground potential were taken simultaneously by the sensors 45, with five successive simultaneous measurements being taken using five different base reference sensors located in bores in the ground at positions indicated by the references 47. The resulting contour map is an amalgam of the five different sets of measurements with the different areas of the map indicated as 50 to 60 inclusive representing the following voltage measurements: 50 minus 16 to minus 10 millivolts 51 minus 10 to minus 4 millivolts 52 minus 4 to 2 millivolts 53 2 to 8 millivolts 54 8 to 14 millivolts 55 14 to 20 millivolts 56 20 to 26 millivolts 57 26 to 32 millivolts 58 32 to 38 millivolts 59 38 to 44 millivolts 60 44 to 50 millivolts Figure 10 shows a similar contour map taken from similar readings but after a trace material (such as salt) was injected into the ground. The change in ground potential in the area indicated as 61 indicates a leak in the wall in this area. Figure 11 shows very diagrammatically a profile plot of apparent resistivity RV as measured by a plurality of sensors spaced along the length L of a wall, the measurements being taken substantially simultaneously with the injection of a current into the ground at the base sensor. The graph is arrived at by cumulating figures from a plurality of readings using differently positioned base sensors on the other side of the wall. Corrections will be made for ground potential and for the length of the travel of the signal to the sensors in the calculations of the resistivity. The dip in the graph indicating a reduction in apparent resistivity indicates that a leak is present in this position of the wall. Where a subsurface wall does not extend up to the ground level, the ground above the wall will be excavated as indicated at 63 in Figure 6 so that current will not pass through the ground above the wall but will tend to find the easiest path through the wall indicating the existence of a hole in that wall.

Claims: CLAIMS 1. A method of collecting and recording geophysical data which comprises recording substantially simultaneously a plurality of first type measurements of the ground potential using sensors at an array of selected positions in a target area, each measurement being relative .to a common base reference, recording substantially simultaneously a plurality of second type measurements of ground potential using the same sensors in the same array and with the same base reference, the first type measurements being made substantially coincidentally with introducing a known current pulse into the ground, and correlating the recorded measurements to produce one or more displays of the variation of electrochemical potential and/or resistance of the ground over the target area. 2. A method according to claim 1 characterised in that the sensors are a plurality of non polarised measuring sensors arranged in a substantially two-dimensional array over the surface of the target area. 3. A method according to claim 1 in which the sensors are located in an array in a bore hole in the ground in the target area. 4. A method according to claim 1 or claim 2 in which a common base reference is located in a bore hole in the ground. 5. A method according to claim 1 which includes making several measurements of the first type and of the second type using substantially the same sensors but using a series of different base references in succession. 6. A method according to claim 5 in which some or each of the sensors located at the selected positions are used in turn to serve as a base reference. 7. A method according to claim 1 or claim 2 in which at least a base reference is outside the target area. 8. A method according to claim 1 in which the sensors at the selected positions are divided into groups, each group being associated with an individual data recording and transmission unit. 9. A method according to claim 8 in which initially the potential of each sensor relative to others of the sensors in its group is measured and the results are surveyed to ensure that each sensor is properly connected both to a control system and to the adjacent ground surface., 10. A method according to claim 1 which includes calculating the resistance or resistivity of the ground from the first type measurements corrected for the measured electrochemical potential at the positions. 11. A method according to claim 10 which includes correcting said calculations of resistivity for the natural variation of the data in different magnetic directions. 12. A method according to claim 1 in which the target area is adjacent a wall, a series of walls or a matrix of walls intended to seal against the flow of water therethrough and the measurements are used to locate the position of leakage through one or more of the walls. 13. A method according to claim 12 which includes artificially introducing a variation in or enhancement of the leak flow pattern through said wall or walls by pumping water to or away from one side of the wall or one area between or outside the walls and taking the recordings before and after the artificial variation. 14. A method according to claim 12 or claim 13 which includes introducing a tracer material into the ground water to highlight the changes in the subsoil. 15. A method according to claim 1 or claim 2 which includes preparing from the recorded data a pair of contour maps, one of resistance and one of electrochemical potential over the substantially horizontal target area. 16. Apparatus for measuring and recording ground resistance and electrochemical potential between a reference position and a plurality of measuring positions in an array in a target area, comprising microprocessor means (12), connected by cable means to a plurality of measuring sensors (15), characterised in that each sensor is capable of measuring the natural electrochemical potential of the ground at a position in the array at which it is located relative to a reference potential sensor (16, 17), the processor means being programmed to control and record substantially simultaneous measurements from all the sensors, the processor means being programmed to identify the information from each individual sensor and relate this to the position of the sensor in the array, and means arranged to use the data from said measurements to produce contour and/or graphic profile displays (Figures 3 and 4) of the variation of the ground resistance and/or electrochemical potential over the target area. 17. Apparatus according to claim 16 characterised by direct voltage supply means for introducing into the ground at the base reference point a known current pulse and ensuring that the sensors simultaneously make a recording of the measured potential substantially coincidentally with the introduction of the current. 18. Apparatus according to claim 17 characterised in that the processor means is arranged to calculate data concerning the ground resistance from the potential measurements taken by the sensors on injection of the current pulse. 19. Apparatus according to claim 18 characterised in that the processor means is arranged to compensate for the measured electrochemical potential of the ground in said calculations. 20. Apparatus according to any of claims 16 to 19 in which the sensors are arranged in groups, each group being associated with an individual data recording and transmission unit (14), and in which each data transmission and recording unit is connected by the cable (13) to the central data receiving and processor means. 21. Maps or displays made using the method or apparatus of any of the preceding claims.

Inventors: SCOTT ROY, SCOTT, ROY
Applicants: APEX DATA SYSTEMS LTD, SCOTT ROY, APEX DATA SYSTEMS LIMITED, SCOTT, ROY
Application year: 1994
Main classification: G01V3
IPC codes: G01V3